The present invention relates to a mould for moulding a container obtained from a parison of plastic material and a method thereof. In particular, said mould finds application in the bottling sector, e.g. in linear or rotary carousel blow-moulding machine.
As it is well-known, the blow-moulding process starts with the insertion of the parison inside the open mould and is completed with the extraction of the moulded container from said mould. These transfers are particularly critical since the parison and the container must be adequately supported.
Traditionally, these transfers are performed by means of two transfer star-wheels (one for transferring the parisons and another for transferring the containers) provided with several stations, each station being equipped with a dedicated gripper for pinching the parison or the container. For example, during the transfer of the parison from the star-wheel to the mould (or the transfer of the container from the mould to the star-wheel), the relative speed and path of the axis of the gripper should coincide with speed and path of the axis of symmetry of the mould (that means the axis of the parison or the container inside the mould) on the tangent point between the trajectory of grippers and moulds. Theoretically, this coincidence takes place in a single point and it lasts an infinitesimal time interval.
Nevertheless, at the state of the art, the transfer may envisage that the gripper moves according to a radial direction of the star-wheel. In alternative, the gripper rotates around an external point with respect to the line connecting the two axis mentioned above. Another known alternative consists in a combination of a linear and rotating movement of the gripper. Since these transfers are driven by mechanical equipments (motors, cams, rods, etc.) that have real timing characteristics, the interval of coincidence of the axis of the gripper with the axis of symmetry of the mould is finite (greater than an infinitesimal time interval). The longer is said interval, the larger is the stroke of the gripper and, thus, the higher is the cost and complexity of the whole machine. Moreover, a large stroke of the gripper determines a bigger pitch-line diameter of the star-wheel and, thus, a higher distance between the star-wheels.
Even more so, said distance between the star-wheels is linked to the dead angle of the blow-moulding machine. Indeed, an increase in the dead angle produces a decrease in the yeld of the blow-moulding process.
Moreover, another disadvantage of prior art solutions is linked to the fact that the parison must be disengaged by the gripper only after the mould has been closed around the parison so as to maintain it. On the other side, the container must be pinched by the gripper when the mould is still closed.
Another drawback, described hereafter, must be pointed out when dealing with prior art solutions. In case of parison with upward-oriented mouth, the parison is maintained in the cavity defined by the mould thanks to the specific enlargement obtained in proximity of the mouth. In case of parison with downward-oriented mouth, the enlargement cannot be used to support the parison since it serves for applying the blowing nozzle. In this case, other dedicated supporting member must be employed.
Prior art solutions are known from EP2113367, U.S. Pat. Nos. 3,311,684, 2,998,621 and FR2874193. Nevertheless, all these solutions suffers of non-instantaneous disengaging of the parison and, in some cases, of bulky design. In this context, the technical task at the basis of the present invention is to provide a mould for moulding a container obtained from a parison of plastic material and method thereof, which overcome the drawbacks of the above mentioned known art.